1. Field of the Invention
The present invention relates to a control method of adaptive array and an adaptive array apparatus for receiving a signal in spatially selective manner employing a plurality of sensors.
2. Description of the Related Art
In a field of obtaining a voice signal, sonar, radio communication and so forth, in order to receive only a specific signal among a plurality of signal sources, a voice enhancing device employing an adaptive microphone array, a radio signal transmitting and receiving device employing an adaptive antenna array and so forth are known as application of an adaptive array technology.
As sensor, a microphone, an ultrasonic sensor, a sonar receiver, antenna and so forth may be employed. Discussion will be given hereinafter for the case where the microphone is used as the sensor.
Here, for simplification of disclosure, consideration is given for the case where the microphones are aligned with equal interval. On the other hand, a target sound source is considered to be located sufficiently distant from a line, on which the microphones are arranged. Also, a situation where the target sound source is arranged perpendicularly to the line on which the microphones are aligned.
A microphone array forms a spatial filter by summing signals sounded by a plurality of microphones after filtering. By such spatial filter, an environmental noise can be suppressed to permit reception of signal arriving from a predetermined direction, namely only a target sound. The adaptive microphone array is a microphone array which adaptively varies spatial filtering characteristics. As constructions of the adaptive microphone array, a construction disclosed in xe2x80x9cGeneralized Side Lobe Cancellerxe2x80x9d, IEEE, Transactions on Antennas and Propagation, Vol. 30, No. 1, 1982, pp 27 to 34 (hereinafter referred to as xe2x80x9cpublication 1xe2x80x9d), a construction disclosed in IEEE, Transactions on Antennas and Propagation, Vol. 40, No. 9, 1992, pp 1093 to 1096 (hereinafter referred to as xe2x80x9cpublication 2xe2x80x9d), a construction disclosed in Paper of The Institute of Electronics. Information and Communication Engineers, Vol. 79, No. 9, 1966, pp 1516 to 1524 (hereinafter referred to as xe2x80x9cpublication 3xe2x80x9d), a construction shown in xe2x80x9cFrost Beam Formerxe2x80x9d, IEEE, Processing of IEEE, Vol. 60, No. 8, 1972, pp 926 to 935, a construction disclosed in IEEE, Processings of International Conference on Acoustics, Speech and Signal Processing 94, 1994, pp IV-267 to 272 (hereinafter referred to as xe2x80x9cpublication 5xe2x80x9d) and so forth are known.
Here, discussion will be given for operation of the construction of the publication 3 as typical construction, with reference to the drawing.
FIG. 35 shows a signal processing portion of an adaptive array of the publication 3, when M microphones are employed. Signals of a microphone group 1m (m=0, 1, . . . , Mxe2x88x921) are converted from analog signals into digital signals, respectively. This digital signal group (hereinafter referred to as microphone signal group) is subject to signal processing to extract a target signal.
The conventional adaptive array device is constructed with a fixed beam former 2, a blocking matrix 20, a multi-input canceller 30. Hereinafter, each of the fixed beam former 2, the blocking matrix 20 and the multi-input canceller 30 will be discussed individually.
As the fixed beam former, a delay and sum beam former which delays and sums signals received from the microphone group and a filter and sum beam former which filters and sums the signals received from the microphone group. Such fixed beam former has been disclosed in D. H. Johnson and D. E. Dudgeon, xe2x80x9cArray Signal Processingxe2x80x9d (Prentice Hall, Englewood Cliffs, 1993, Chapter 4 (hereinafter referred to as xe2x80x9cpublication 6xe2x80x9d). Here, the operation will be discussed in terms of the delay and sum beam. The delay and sum beam former can be expressed by the following expression (1).                               g          ⁢                      xe2x80x83                    ⁢                      (            k            )                          =                              ∑                          m              =              0                                      M              -              1                                ⁢                      xe2x80x83                    ⁢                                    f              m                        ⁢                          xe2x80x83                        ⁢                          x              m                        ⁢                          xe2x80x83                        ⁢                          (                              k                -                                  r                  m                                            )                                                          (        1        )            
wherein k is a sample number in a time axis, and rm is a delayed sample number of respective microphone signals xm(k). g(k) is an output signal of the fixed beam former 2, xm(k) is an output signal of a microphone Im, fm is a coefficients corresponding to the microphone signal in the fixed beam former.
The delay and sum beam former calculates and outputs a sum of the signals multiplied with the coefficients fm with delaying respective microphone signals xm(k) for rm samples. Each delay period rm is set to synchronize the phase of the target signal in a signal xm (kxe2x88x92rm) which is generated by delaying output signals of respective microphones Im. As a result, upon summing xm (kxe2x88x92rm) (m=0, 1, . . . , Mxe2x88x921), the target signal is enhanced. On the other hand, interference signals arrive from directions other than target signal. In the signal xm (kxe2x88x92rm) which is generated by delaying the output signal of each microphone, a phase is significantly different. Upon summing, the signals are canceled with each other to attenuate. Accordingly, in the output of the fixed beam former, the target signal is enhanced and the interference signal is attenuated.
Next, the blocking matrix 20 will be discussed with reference to FIG. 35.
The blocking matrix 20 is constructed with a fixed beam former 3, a delay group 4m(m=0, 1, Mxe2x88x921), an adaptive filter group 5m(m=0, 1, . . . , Mxe2x88x921), a subtractor group 6m(m=0, 1, . . . , Mxe2x88x921). The blocking matrix 20 is employed for adaptive signal processing to transmit a signal group, in which the target signal is attenuated and the signals other than the target signal are enhanced to the multi-input canceller 30.
As a filter structure of the adaptive filter group 5m of the blocking matrix 20, a finite impulse response (FIR) filter, an infinite impulse response (IIR) filter, lattice filter and so forth can be employed. Here, discussion will be given for the case where the FIR filter is employed.
The fixed beam former 3 receives the signal group from the microphone 1 and outputs a signal, in which the target signal is enhanced and the interference signals are attenuated by signal processing similar to the fixed beam former 2. The output signal of the fixed beam former 3 becomes an input signal common to the adaptive filters 5m. Each delay 4m receives the output signal of corresponding one of the microphone 1m to transmit the delayed signal to corresponding one of the subtractor 6m. Each of the adaptive filters 5m receives the output signal of the fixed beam former 3 and transmits an output signal, in which a tap coefficients contained in the corresponding adaptive filter 5m is convoluted, to the corresponding subtractor 6m. Each subtractor 6m subtracts the output signal of the corresponding adaptive filter 5m from the output signal of the corresponding delay 4m. A result of subtraction of the subtractor 6m is transmitted to the corresponding adaptive filter 7m of the multi-input canceller 30 as the output signal of the blocking matrix 20, and, in conjunction therewith, to the corresponding adaptive filter 5m for updating the tap coefficients.
A delay period of the delay 4m is set so that phases of a target signal component in the output of the delay 4 m and a target signal component in the output of the adaptive filter 5m are consistent with each other. For example, the delay period of the delay 4m may be set at a period as a sum of a group delay period of the fixed beam former 3 and a period of about one fourth to one half of a period corresponding to a tap number of the adaptive filter 5m. 
The process in the blocking matrix can be expressed by the following expression (2)
ym(k)=xm(kxe2x88x92P)xe2x88x92HmT(k)D(k)(m=0,1, . . . , Mxe2x88x921)xe2x80x83xe2x80x83(2)
wherein ym(k) is an output signal of the subtractor group 6m, xm(kxe2x88x92P) is an output signal of the delay 4m, P is a delay period of the delay 4m. Also, {xc2x7}T represents transpose. Hm(k) is a tap coefficients vector of the adaptive filter 5m, D(k) is a signal vector consisted of a plurality of signals generated by delaying the output signal d(k) of the fixed beam former 3 which is expressed as the following expressions (3) and (4) assuming that the tap number is N.
xe2x80x83Hm(k)=[hm,o(k), hm,1(k), . . . hm,Nxe2x88x921(k)]T(m=0,1, . . . , Mxe2x88x921)xe2x80x83xe2x80x83(3)
D(k)=[d(k), d(kxe2x88x921), . . . , d(kxe2x88x92N+1)]Txe2x80x83xe2x80x83(4)
Hereinafter, detailed discussion of the adaptive filter 5m will be given.
As the adaptive filter 5m, updating of the tap coefficients is performed so that an output signal power of the subtractor 6m can be minimized. Hereinafter, updating of the coefficients in the adaptive filter 5m will be discussed in detail.
As an algorithm adapted for updating the tap coefficients in the adaptive filter 5m, an NLMS algorithm (also called as xe2x80x9clearning identification methodxe2x80x9d), RLS algorithm (also called as xe2x80x9crecursive least square methodxe2x80x9d), a projection algorithm, a gradient method, a least square method, a block adaptive algorithm, adaptive algorithm employing orthogonal transformation and so forth may be used. A discussion will be given hereinafter for the case where NLMS algorithm is employed as the adaptive algorithm.
Updating of the tap coefficients Hm(k) (m=0, 1, . . . ) Mxe2x88x921) of the leak adaptive filter employing the NLMS algorithm can be expressed by the following expressions (5) and (6).                                                         H              m                        ⁢                          xe2x80x83                        ⁢                          (                              k                +                1                            )                                =                                                    H                m                            ⁢                              xe2x80x83                            ⁢                              (                k                )                                      -                          δ              ⁢                              xe2x80x83                            ⁢                              H                m                            ⁢                              xe2x80x83                            ⁢                              (                k                )                                      +                          β              ⁢                              y                m                            ⁢                              xe2x80x83                            ⁢                                                (                  k                  )                                                                      "LeftDoubleBracketingBar"                                          D                      ⁢                                              xe2x80x83                                            ⁢                                              (                        k                        )                                                              "RightDoubleBracketingBar"                                    2                                            ⁢                              xe2x80x83                            ⁢              D              ⁢                              xe2x80x83                            ⁢                              (                k                )                                                    ⁢                  
                ⁢                  (                                    m              =              0                        ,            1            ,            …            ⁢                          xe2x80x83                        ,                          M              -              1                                )                                    (        5        )                                0         less than         β         less than         2                            (        6        )            
wherein xcex2 is a constant determining a step size, xcex4 is leaky coefficients. In updating of the tap coefficients, the constant xcex2 determining the step size becomes the step size as is.
Updating of the tap coefficients of each adaptive filter 5m is performed for minimizing the power of the signal in the output of the corresponding subtractor 6m. The output signal of the fixed beam former 3 as the input signal of the adaptive filter 5m mainly contain the target signal. Therefore, as a result of minimization of the output signal power of each subtractor 6m, the target signal is significantly attenuated at the output of the subtractor 6m.
Subsequently, discussion will be given with respect to the multi-input canceller 30.
The multi-input canceller 30 receives the output signal of the fixed beam former 2 and the output signal group of the blocking matrix 20 to eliminate a component correlated with the output signal group of the blocking matrix 20 from the signal derived by delaying the output of the fixed beam former 2.
The multi-input canceller 30 is constructed with adaptive filter group 7m (m=0, 1, . . . ) Mxe2x88x921) corresponding to the output signals of respective subtractor group 6m, an adder 8m a delay 11 and a subtractor 9.
Each adaptive filter 7m receives the output signal of corresponding subtractor 6m to transmit filter coefficients and convolution result to the adder 8 as the output signal. As a filter structure of the adaptive filter 7m, an FIR filter, an IIR filter, a lattice filter and so forth can be employed. Here, discussion will be given hereinafter for the case where the FIR filter is employed.
The adder 8 receives all of the outputs of the adaptive filter group 7m to derive a sum of the received signal for outputting as a result to the subtractor 9.
The delay 11 receives the output signal of the fixed beam former 2 and transmits the delayed signal to the subtractor 9.
The subtractor 9 subtracts the output signal of the adder 8 from the output signal of the delay 11. The output signal of the subtractor 9 becomes the output of the multi-input canceller 30, namely the output of the overall adaptive array apparatus.
A delay period Q of the delay 11 is set to make the phase of the signals to be consistent in the output of the adaptive filter 7m and the output of the delay 11 with respect to arbitrary direction of arrival. For example, the delay period Q is set to a period from one fourth to three fourth of tap number of the adaptive filter 7m. 
The process in the multi-input canceller 30 can be expressed by the following expression (7).                               z          ⁢                      xe2x80x83                    ⁢                      (            k            )                          =                              g            ⁢                          xe2x80x83                        ⁢                          (                              k                -                Q                            )                                -                                    ∑                              m                =                0                                            M                -                1                                      ⁢                          xe2x80x83                        ⁢                                          W                m                T                            ⁢                              xe2x80x83                            ⁢                              (                k                )                            ⁢                              xe2x80x83                            ⁢                              Y                m                            ⁢                              xe2x80x83                            ⁢                              (                k                )                                                                        (        7        )            
wherein g(kxe2x88x92Q) is an output of the delay 11, namely as a signal derived by delaying the output signal of the fixed beam former 2. On the other hand, z(k) is the output signal of the subtractor 9, namely the output signal of the overall adaptive array. Also, Wm(k) is a tap coefficients vector of the adaptive filter 7m, Ym(k) is the input signal of the adaptive filter 7m, namely a signal vector consisted of a signal derived by delaying the output signal ym(k) of the blocking matrix. Wm(k) and Ym(k) are defined as following expressions (8) and (9) with taking the tap number being L.
Wm(k)=[wm,0(k), wm,1(k), . . . , wm,Lxe2x88x921(k)]Txe2x80x83xe2x80x83(8)
Ym(k)=[ym(k), ym(kxe2x88x921), . . , ym(kxe2x88x92L+1)]T(m=0,1, . . . , Mxe2x88x921)xe2x80x83xe2x80x83(9)
In each adaptive filter 7m, on the basis of the output signal of the subtractor 9, the tap coefficients Wm(k) is updated so that the power of the output signal z(k) of the subtractor 9 is minimized. The adaptive filter 7m will be discussed hereinafter in detail.
As the adaptive filter 7m, a tap coefficients constrained adaptive filter or leaky adaptive filter may be employed. Here, discussion will be given for the case where the leaky adaptive filter is employed.
As adaptive algorithm for updating the tap coefficients in the adaptive filter 7m, the NLMS algorithm, the RLS algorithm, the projection algorithm, the gradient method, the least square method, the block adaptive algorithm, the adaptive algorithm employing orthogonal transformation and so forth may be used. A discussion will be given hereinafter for the case where NLMS algorithm is employed as the adaptive algorithm.
When the NLMS algorithm is applied as the adaptive algorithm, there are a method applying NLMS algorithm per each adaptive filter and a method applying NLMS algorithm aggregatingly for all adaptive filter group. Here, discussion will be given for the method applying NLMS algorithm aggregatingly for all adaptive filter group.
Updating of coefficients of the tap coefficients Wm(k) in the adaptive filter group 7m can be expressed by the following expression (10) with taking the constant determining the step size being xcex1.                                                         W              m                        ⁢                          xe2x80x83                        ⁢                          (                              k                +                1                            )                                -                                    W              m                        ⁢                          xe2x80x83                        ⁢                          (              k              )                                -                      γ            ⁢                          xe2x80x83                        ⁢                          W              m                        ⁢                          xe2x80x83                        ⁢                          (              k              )                                +                      α            ⁢                          xe2x80x83                        ⁢                                          z                ⁢                                  xe2x80x83                                ⁢                                  (                  k                  )                                                                              ∑                                      m                    =                    0                                                        M                    -                    2                                                  ⁢                                  xe2x80x83                                ⁢                                                      "LeftDoubleBracketingBar"                                                                  Y                        m                                            ⁢                                              xe2x80x83                                            ⁢                                              (                        k                        )                                                              "RightDoubleBracketingBar"                                    2                                                      ⁢                          xe2x80x83                        ⁢                          Y              m                        ⁢                          xe2x80x83                        ⁢                          (              k              )                                      ⁢                  
                ⁢                  (                                    m              =              0                        ,            1            ,            …            ⁢                          xe2x80x83                        ,                          M              -              1                                )                                    (        10        )            
wherein ||xc2x7|| is an Euclidean norm, xcex3 is a leaky coefficients. In updating of coefficients, the constant xcex1 determining the step size becomes the step size as is.
Updating of the tap coefficients of the adaptive filter group 7m is performed for minimizing the output of the signal at the output of the subtractor 9. The output signal of the subtractor 6m as the input signal of each adaptive filter 7m mainly contain the interference signal. Therefore, as a result of minimization of the output signal power of the subtractor 9, the interference signal is significantly attenuated in the output of the subtractor 9.
The conventional adaptive array apparatus set forth above can extract the target signal in the under presence of the interference signal.
In the conventional adaptive array apparatus, when only target signal is present and the interference signal is not present, the tap coefficients Wm(k) in the adaptive filter group 7m of the multi-input canceller 30 can be disturbed by the target signal to cause degradation of the target signal at the final output. On the other hand, when the interference signal is generated again after disturbance of the filter coefficients Wm(k), the interference signal cannot be removed to cause breathing noise in the final output.
For presenting degradation or noise, it becomes necessary to set the constant xcex1 for determining the step size of the adaptive algorithm small so as not to disturb the adaptive filter group 7m of the multi-input canceller 30. However, when xcex1 is small, a following speed of the adaptive filter group 7m in the multi-input canceller 30 relative to movement of the interference signal source, becomes low to elongate period, in which removal of the interference signal is unsatisfactory in the final output.
On the other hand, when only interference signal is present and the target signal is not present, the filter coefficients Hm(k) in the adaptive filter group 5m of the blocking matrix 20 is disturbed to cause breathing noise in the final output. In order to prevent this, it becomes necessary to make the constant xcex2 determining the step size of the adaptive algorithm small in order to avoid disturbance of the adaptive filter 5m. However, when xcex2 is small, following speed of the adaptive filter group 5m in the blocking matrix 20 with respect to movement of the target signal source, becomes low to cause degradation of quality of the target signal in the final output.
In the conventional adaptive array apparatus, in order to make breathing noise smaller or to make the quality of the output signal higher, it becomes necessary to set the constant determining the step size smaller in the adaptive filter of the multi-input canceller. On the other hand, for making following speed for movement of the interference signal source high, the constant has to be set large. Thus, in the conventional adaptive array apparatus, in setting of the constant determining the step size in the adaptive filter of the multi-input canceller, reducing of the breathing noise, enhancing quality of the output signal and making the following speed with respect to movement of the interference signal source high are in a relationship of trade-off.
On the other hand, in the conventional adaptive array apparatus, it becomes necessary to make the constant determining the step size in the adaptive filter of the blocking matrix small. On the other hand, in order to obtain high following speed for movement of the target signal source, the constant has to be set greater. Thus, in the conventional adaptive array apparatus, in setting of the constant determining the step size in the adaptive filter of the blocking matrix, reducing of the breathing noise and making the following speed with respect to movement of the interference signal source high are in a relationship of trade-off.
It is an object of the present invention to provide a control method of adaptive array and an adaptive array apparatus which can obtain high following speed with respect to movement of an interference signal source with maintaining a breathing noise small and quality of an output signal high.
According to the first aspect of the present invention, a control method of an adaptive array employing an adaptive filter for receiving a particular signal source as a target signal source, among a plurality of signal sources, comprises the steps of:
using an indicative value relating to an amplitude of an output signal of a beam former having higher sensitivity with respect to the target signal source than a sensitivity with respect to other signal source;
using an indicative value relating to an amplitude of an output signal of a beam former having lower sensitivity with respect to the target signal source than a sensitivity with respect to other signal source; and
determining a step size of an adaptive algorithm in the adaptive filter.
The present invention in the foregoing first aspect uses the indicative value relating to an amplitude of an output signal of a beam former having higher sensitivity with respect to the target signal source than a sensitivity with respect to other signal source and the indicative value relating to an amplitude of an output signal of a beam former having lower sensitivity with respect to the target signal source than a sensitivity with respect to other signal source for determining the step size of the adaptive algorithm in the adaptive filter.
Accordingly, even when a constant xcex1 determining the step size of the adaptive filter is set large, degradation of the signal and breathing noise in the final output can be suppressed and breathing noise due to movement of the interference signal source can be suppressed with higher following speed with respect to movement of the interference signal source.
According to the second aspect of the present invention, an adaptive array apparatus employing an adaptive filter for receiving a particular signal source as a target signal source, among a plurality of signal sources, comprises:
means for deriving a first indicative value relating to an amplitude of an output signal of a beam former having higher sensitivity with respect to the target signal source than a sensitivity with respect to other signal source;
means for deriving a second indicative value relating to an amplitude of an output signal of a beam former having lower sensitivity with respect to the target signal source than a sensitivity with respect to other signal source; and
means for determining a step size of an adaptive algorithm in the adaptive filter on the basis of a radio of the first and second indicative values.
According to the third aspect of the present invention, an adaptive array apparatus employing an adaptive filter for receiving a particular signal source as a target signal source, among a plurality of signal sources, comprises:
means for deriving a first indicative value relating to an amplitude of an output signal of a beam former having higher sensitivity with respect to the target signal source than a sensitivity with respect to other signal source,
means for deriving a second indicative value relating to an amplitude of an output signal of a beam former having lower sensitivity with respect to the target signal source than a sensitivity with respect to other signal source;
means for comparing the first indicative value and an value derived by multiplying the second indicative value with a constant; and
means for determining a step size of an adaptive algorithm in the adaptive filter on the basis of a result of comparison of the first and second indicative values.
The third aspect of the present invention set forth above compares the first indicative value and an value derived by multiplying the second indicative value with a constant of the second indicative value to determine the step size of the adaptive algorithm of the adaptive filter.
Accordingly, by setting a constant by deriving an value derived by multiplying the second indicative value with a constant, a threshold value for determining whether the filter coefficients is to be updated or not, can be set.
According to the fourth aspect of the present invention, an adaptive array apparatus having a generalized side lobe canceller type construction and having an adaptive filter for receiving only specific signal source as a target signal source among a plurality of signal sources, comprises:
means for deriving a first indicative value relating to an amplitude of an output signal of a beam former having higher sensitivity with respect to the target signal source than a sensitivity with respect to other signal source;
means for deriving a second indicative value relating to an amplitude of an output signal of a beam former having lower sensitivity with respect to the target signal source than a sensitivity with respect to other signal source;
means for deriving a step size determining value proportional to a value derived by converting a quotient of division of the first indicative value by the second indicative value with a non-linear function; and
means for determining a step size of an adaptive algorithm in an adaptive filter provided in a multi-input canceller on the basis of the step size determining value.
In the construction set forth above, with a step size determining value proportional to a value derived by converting a quotient of division of the first indicative value by the second indicative value with a non-linear function, the step size of an adaptive algorithm in an adaptive filter is determined on the basis of the step size determining value.
Accordingly, when the target signal is sufficiently large and the interference signal is sufficiently small, the quotient by dividing the first indicative value by the second indicative value becomes a positive value close to zero to increase period to perform updating of the tap coefficients to make converging speed of the adaptive filter higher.
In the preferred construction, the non-linear function is a step function.
In the alternative, the non-linear function may be a diode function.
In the further alternative, the non-linear function may be a polynomial function.
Also, in the preferred construction, each of the first and second indicative values relating to the signal amplitude is an average value of square of the signal.
In the alternative, each of the first and second indicative values relating to the signal amplitude may be an average value of absolute value of the signal.
In the further alternative, each of the first and second indicative values relating to the signal amplitude is an average value of a given order power of the signal.
According to the fifth aspect of the present invention, a control method for an adaptive array apparatus employing an adaptive filter and an iterative least square algorithm for receiving a particular signal source as a target signal source, among a plurality of signal sources, comprises the steps of:
deriving a first indicative value relating to an amplitude of an output signal of a beam former having higher sensitivity with respect to the target signal source than a sensitivity with respect to other signal source,
deriving a second indicative value relating to an amplitude of an output signal of a beam former having lower sensitivity with respect to the target signal source than a sensitivity with respect to other signal source; and
determining a step size of an adaptive algorithm in the adaptive filter and a forgetting constant on the basis of the first and second indicative values.
According to the sixth aspect of the present invention, an adaptive array apparatus employing an adaptive filter and an recursive least square algorithm for receiving a particular signal source as a target signal source, among a plurality of signal sources, comprises:
means for deriving a first indicative value relating to an amplitude of an output signal of a beam former having higher sensitivity with respect to the target signal source than a sensitivity with respect to other signal source,
means for deriving a second indicative value relating to an amplitude of an output signal of a beam former having lower sensitivity with respect to the target signal source than a sensitivity with respect to other signal source; and
means for determining a step size of an adaptive algorithm in the adaptive filter and a forgetting constant on the basis of the first and second indicative values.
With the invention set forth above, the present invention is applicable for the adaptive filter having the recursive least square algorithm using the forgetting constant.